In order to illustrate an example of an application of the invention, a description is given below of a modem used in reception in a satellite transmission system.
FIG. 1 is a block diagram of a reception system for satellite transmission, the receiver used including a detector for detecting the presence or the absence of a carrier.
The reception system shown in FIG. 1 includes an antenna 10 receiving a microwave signal constituted by a carrier in KU band (12 GHz) modulated by a digital signal, e.g. in 4-PSK. The signal picked up by the antenna 10 is applied to a low noise block (LNB) converter 11 conventionally comprising a low noise amplifier followed by apparatus for performing transposition into intermediate frequency. For example, the output signal from the LNB 11 may be in BIS band (950 MHz to 1,750 MHz) and it is applied to a frequency transposition stage 12 of the modem.
The stage 12 includes an amplifier 13 followed by a mixer 14 receiving a transposition signal from a frequency synthesizer 15. The mixer 14 and the frequency synthesizer 15 constitute a first frequency transposition stage. The output signal from the mixer 14 has a fixed frequency, as explained below, and it is applied to a band-pass filter 16 followed by automatic gain control (AGC) apparatus 17. The output signal from the AGC apparatus 17 is applied to a second frequency transposition stage constituted by a mixer 18 driven by a local oscillator 19. The output signal from the mixer 18 is an intermediate frequency (IF) signal at 70 MHz. This signal is then applied to the demodulation stage of the receiver, which stage serves to restore the transmitted 4-PSK symbols.
The output signal from the AGC apparatus 17 is also applied both to automatic frequency control (AFC) apparatus 20 driving the frequency synthesizer 15, and to apparatus 21 for detecting the presence or the absence of a carrier. The AFC apparatus 20 serves to compensate any frequency drift due to the frequency transposition apparatus included in the LNB 11. The frequency transposition apparatus commonly used in such an LNB includes a local oscillator constituted by a resonator. Such an oscillator may exhibit considerable frequency drift around its central frequency, which is why it is necessary to use AFC apparatus downstream so as to guarantee that the frequency at the output of the mixer 14 is constant.
The apparatus 21 for detecting the presence or the absence of a carrier serves to inhibit operation of the AFC apparatus 20 when absence of carrier is detected so that the synthesizer 15, which is advantageously broad-band, does not latch onto an intermediate frequency corresponding to that of an adjacent channel. In satellite transmissions, a plurality of carriers are used simultaneously at the same symbol rate R, and, to comply with the Intelsat Standard IESS-308, two adjacent channels must be capable of having bands that overlap each other in part (difference between carriers equal to 0.7 R). Furthermore, one of the channels adjacent to the received channel must be capable of having amplitude of up to 7 dB more than that of the received channel.
Under these circumstances, if the AFC apparatus 20 is not inhibited when the carrier disappears, e.g. as a result of temporary selective fading, the AFC apparatus, which is broad-band, re-synchronizes the synthesizer 15 on one of the adjacent channels. The carrier cannot then be subsequently recovered and the link is cut off. That is why the apparatus 21 for detecting the presence or the absence of a carrier generates an alarm signal (ALARM) inhibiting operation of the AFC apparatus 20 when absence of carrier is detected.
In known manner, the presence or the absence of a carrier is detected by detecting the IF level, i.e. the energy of the signal output by the AGC apparatus 17 is compared with a reference value. If the reference value is exceeded, the carrier is considered to be present, and if the reference value is not exceeded, the carrier is considered to be absent.
Unfortunately, detecting a carrier by means of IF detection poses a problem when the dynamic range of the AGC apparatus 17 placed upstream is greater than the signal-to-noise ratio of the received signal. In the absence of carrier, the AGC apparatus 17 amplifies the noise level to the level of the absent carrier, and the alarm signal is not generated. In which case, the AFC apparatus 20 is not inhibited and the synthesizer 15 latches onto the intermediate frequency corresponding to an adjacent channel.
Moreover, even in the absence of adjacent channels, absence of detection of carrier loss can have considerable effects on receiver operation. By way of example, in a TDMA transmission application, the receiver receives information packets spaced apart over time. In order to demodulate the information, the receiver must know the envelope of the received signal, i.e. the moments at which the packets start and end. In the state of the art, this timing information is obtained in predictive manner. The drawback with that is that if packet transmission stops, e.g. as a result of transmitter breakdown, the AGC apparatus amplifies noise, the IF carrier-presence detector does not operate, and the AFC apparatus is not inhibited. As a result, the TDMA system loses synchronization.